Is there a reason why a base class decorated with XmlInclude would still throw a type unknown exception when serialized?

I will simplify the code to save space but what is presented does illustrate the core problem.

I have a class which has a property that is a base type. There are 3 derived classes which could be assigned to that property.

If I assign any of the derived classes to the container and attempt to serialize the container, the XmlSerializer throws the dreaded:

"The type x was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically."

However my base class is already decorated with that attribute so I figure there must be an additional "hidden" requirement.

The really odd part is that the default WCF serializer has no issues with this class hierarchy.

The Container class

[DataContract]
[XmlRoot(ElementName = "TRANSACTION", Namespace = Constants.Namespace)]
public class PaymentSummaryRequest : CommandRequest
{
    [DataMember]
    public PaymentSummary Summary { get; set; }

    public PaymentSummaryRequest()
    {
        Mechanism = CommandMechanism.PaymentSummary;
    }
}

The base class

[DataContract]
[XmlInclude(typeof(xPaymentSummary))]
[XmlInclude(typeof(yPaymentSummary))]
[XmlInclude(typeof(zPaymentSummary))]
[KnownType(typeof(xPaymentSummary))]
[KnownType(typeof(yPaymentSummary))]
[KnownType(typeof(zPaymentSummary))]
public abstract class PaymentSummary
{     
}

One of the derived classes

[DataContract]
public class xPaymentSummary : PaymentSummary
{
}

The serialization code

var serializer = new XmlSerializer(typeof(PaymentSummaryRequest));
serializer.Serialize(Console.Out,new PaymentSummaryRequest{Summary = new xPaymentSummary{}});

The Exception

System.InvalidOperationException: There was an err